Tridymite, SiO

Crystalline Silica. Sihca exists in a variety of polymorphic crystalline forms (23,41—43), in amorphous modifications, and as a Hquid. The Hterature on crystalline modifications is to some degree controversial. According to the conventional view of the polymorphism of siHca, there are three main forms at atmospheric pressure quart2, stable below about 870°C tridymite, stable from about 870—1470°C and cristobaHte, stable from about 1470°C to the melting point at about 1723°C. In all of these forms, the stmctures are based on SiO tetrahedra linked in such a way that every oxygen atom is shared between two siHcon atoms. The stmctures, however, are quite different in detail. In addition, there are other forms of siHca that are not stable at atmospheric pressure, including that of stishovite, in which the coordination number of siHcon is six rather than four. 

If all the four corners of the SiO tetrahedra are shared, three-dimensional networks result. The different forms of silica (quartz, tridymite and cristobalite) discussed earlier in this section are typical examples. Feldspars are generated when part of the Si is... 

Fig. 8.2. Silicon K x-ray emission spectra of a-quartz (solid line), (3-tridymite (dotted curve), and v-SiO, ( suprasil dashed curve) (after Wiech and Kurmaev, 1985 reproduced with the publisher s permission).

Silicon dioxide (sihca), SiO. , occurs in nature in three different crystal forms, as the minerals quartz (hexagonal), cristohalite (cubic), and tridymite (hexagonal). Quartz is the most widespread of these minerals it occurs in many deposits as well-formed hexagonal crystals, and also as a clrystalline constituent of many rocks, such as granite. It is a hard, colorless substance, with hardness 7 on the Mohs scale. Its crystals may be identified as right-handed or left-handed, by their face development (Fig. 31-1), and also by the direction in which they rotate the plane of polarization of polarized light. 

Cristobalite and tridymite are similarly made from SiO tetrahedra fused together by sharing oxygen atoms, with, ho vever, different arrangements of the tetrahedra in space than that of quartz. 

Silica, in its various forms, is a three-dimensional framework of SiO units which share all their corners. Cristobalite for instance has Si04 tetrahedra arranged as are the atoms in zinc blende tridymite as are those in wurtzite. 

Cristobalite (SiO ) / tridymite (8102) derived, respectively, from a network of silicon atoms, zinc-blende, or wurtzite, with one atom of oxygen situated in the middle of each Si-Si segment BeF2 of cristobalite structure. 

Tectosilicates and feldspars If in the tridimensional framework corresponding to the stoichiometric formula SiO (e.g., tridymite or cristobalite) a certain proportion of silicium atoms is substituted by aluminum atoms, in order for the lattice to preserve its structure, it is necessary that the crystal acquire one supplementary electron per aluminum atom. In order to restore the electroneutrality, the crystals accept ions, e.g., Na, which penetrate the holes of the lattice and become linked to the oxygen atoms. 

It has been suggested (Ref. 8, p. 246) that the silica surface is charged by hydroxyl ions formed by the loss of protons from water molecules located in the spaces between the oxygen atoms of the SiO structure (Figure 1.11). Thus the negative charge would lie just within the surface. If the surface resembles that of tridymite on a local scale, as appears possible since it is consistent with 4.6 SiOH groups per square nanometer, then there are depressions in which the counter cations could remain close to the counterions. 

It is interesting to note that AlPO crystals of the tridymite-type generally show defects in the lattice structure. AlPQ, however, crystallizes more easily into the ideal two-layer structure of tridymite than does Si02. The reason is the higher polarization of the phosphate tetrahedron compared with that of the (SiO l -tetrahedron. 

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